This invention relates to a method of applying a program voltage for an electrically erasable non-volatile semiconductor memory (EEPROM) and an apparatus for implementing the same, and more particularly to a method of applying a program voltage for a non-volatile semiconductor memory used in the case of carrying out injection of electrons into an interface level between a floating gate, or SiO.sub.2 (silicon oxide) and Si.sub.3 N.sub.4 (silicon nitride) or extraction therefrom by a Fowler-Nordheim current through an insulating film, and an apparatus for implementing the same.
Referring to FIG. 1, among EEPROMs of this type, there is shown a typical structure of a FLOTOX (floating-gate tunnel oxide) type EEPROM. Namely, on the substrate surface between the source S and the drain D, a stacked layer gate consisting of a control gate CG and a floating gate FG and a select gate SG are provided. At a portion of the insulating film OX below the floating gate FG, a tunnel insulating film TOX for permitting injection/extraction into/from the floating gate FG is provided.
The condition for applying a voltage to respective terminals in carrying out injection/extraction of electrons into/from the floating gate FG of the EEPROM of the structure as described above is shown in Table 1.
TABLE 1 ______________________________________ Control Select Gate Gate Drain Source ______________________________________ Electron GND V.sub.PP V.sub.PP Open Extraction Electron V.sub.PP V.sub.PP GND Open Injection ______________________________________
In this instance, V.sub.PP is called a program voltage, which is set a value (e.g., approximately 15 to 20 V) higher than an applied voltage at the time of data read operation. In Table 1, in injecting electrons, a program voltage V.sub.PP is applied to the control gate CG and the select gate SG, the drain D is grounded, and the source is caused to be in an open state. On the other hand, in extracting electrons, a program voltage V.sub.PP is applied to the select gate SG and the drain D, the control gate CG is grounded, and the source S is caused to be in an open state.
By carrying out injection/extraction of electrons into/from the floating gate FG as stated above, a threshold value where the channel between the source S and the drain is turned on is changed. By such a change in the threshold value, discrimination between writing ("1") into the EEPROM and erasing ("0") the content thereof is conducted.
Meanwhile, a trapezoidal pulse which is blunt in rise (i.e., having a predetermined gradient) is used as the waveform of a program voltage VG.sub.PP applied in injecting electrons into the above-mentioned EEPROM and extracting them therefrom. FIG. 2A shows a waveform of a program voltage V.sub.PP used in the prior art. Ordinarily, its rise time is about 10 .mu.s and its pulse width W is several ms.
When a trapezoidal pulse as shown in FIG. 2A is applied to the control gate CG, etc. as a program voltage, an electric field having a waveform as shown in FIG. 2B is produced in the tunnel oxide film TOX. Further, a Fowler-Nordheim current having a waveform approximate to a rectangular waveform as shown in FIG. 2C flows in the region of a fixed electric field indicated by slanting lines of the electric field waveform. Most of this Fowler-Nordheim current flows for a rise period of the control gate voltage shown in FIG. 2A. When the control gate voltage has reached the program voltage V.sub.PP, injection/extraction of electrons are substantially completed.
When a Fowler-Nordheim current flows in the tunnel oxide film TOX, electrons flowing thereinto are subjected to impact ionization, thus to generate positive holes. A portion of holes generated in the tunnel oxide film TOX as stated above are subjected to recombination with electrons and extinguished. Remaining greater part of holes flow out toward the outside of the tunnel oxide film TOX by an electric field applied to the tunnel oxide film TOX. At this time, some holes remain at a trap level in the tunnel oxide film TOX with a fixed probability as they are. Such a phenomenon is called a trap of holes.
FIG. 2D shows density per unit volume of holes generated in the tunnel oxide film TOX as a result of the fact that a rectangular current pulse as shown in FIG. 2C, in a fixed electric field region as shown by slanting lines of FIG. 2B, flows in the tunnel oxide film TOX.
In FIG. 2D, the total number of electrons per unit volume is indicated by an area (a value obtained by integrating hole density in terms of time) defined by the abscissa and the curve. The value obtained by the value of the area by the probability where holes are trapped becomes equal to the total number per unit volume of holes trapped by one injection/extraction of holes. Every time injection/extraction into/from EEPROM is executed, holes trapped in the tunnel oxide film TOX are stored. When the number of holes stored exceeds above a certain limit, the tunnel oxide film TOX is subjected to dielectric breakdown. As a result, injection/extraction of electrons becomes impossible. This phenomenon gives a main cause determining the life time of a memory cell.